Fracture-cave reservoirs mainly exist in holes, caves and fractures. Karst caves and fractures are the main reservoir spaces of fracture-cave reservoirs. Fractures are the main channels that communicate karst caves. The flow characteristics of formation fluids of fracture-cave reservoirs are much different from those of sandstone reservoirs. The remaining oil in formations of fracture-cave reservoirs is rich, but the sweep volume of injection water is small and the recovery ratio is very low.
At present, there are mainly three methods for flow modification in fracture-cave reservoirs: the first method is to inject a gas (CO2 or N2) or light fluid to attain a purpose of sweeping the crude oil at higher positions; the second method is to plug dominant flow channels with solidified particles, i.e., inject solidified particles to plug dominant flow channels around the well, so as to attain a purpose of modifying the flows and improving the swept volume; the third method is to borrow the plugging modification method for sandstone reservoirs, i.e., inject a gel plugging agent to improve the swept volume. Gas injection can only solve the problem of vertical sweeping in fracture-cave reservoirs, but is difficult to improve planar sweeping; solidified particles are difficult to enter into the deep strata, have high density, are susceptible to dilution by formation water, and can only act on areas in the vicinity of the well; gel systems are susceptible to dilution by cavern water, are hard to build up structural strength in deep strata, have limited temperature-resistance and salinity-resistance properties, and consequently are difficult to use for deep flow modification for demanding high-temperature high-salinity fracture-cave reservoirs.
In China, fracture-cave reservoirs are the most abundant in the Western China region, where the formation conditions are harsh and the formation temperature is usually higher than 90° C. Especially, in some new reservoir blocks, the temperature is as high as 180° C. and the salinity is higher than 20×104 mg/L. Consequently, conventional plugging agents can't meet the requirements of oil development and application.